The ability of rotator cuff tendons to heal back to bone following injury is limited, and failure of surgically repaired rotator cuff tears in humans has been reported in 20-70% of cases. Many factors outside of the surgeon's control contribute to the limited healing potential including patient age, tear size, and time from injury to repair. However, two important factors that are within the surgeon's control are surgical repair technique and post-operative rehabilitation protocol. While much research has been done on surgical repair technique, surprisingly, very little data in regard to post-operative rehabilitation protocols following tendon to bone repairs in the shoulder are available to guide clinicians. As a result, the current clinical trend to passively mobilize the shoulder shortly after repair, which is fraught with a high incidence of failure currently, has relied on data from tendon to tendon healing in the hand. However, data from our animal model and that of others indicates that the response of tendon to activity may be different when healing to bone rather than to tendon. We recently developed an animal model in which healing of the rotator cuff tendon to bone insertion site could be carefully evaluated as a function of post-operative activity level. Immobilization was found to result in better healing than either cage activity or exercise and the longer the period of immobilization, the better the insertion site properties. Based on these results, we now hypothesize that remobilization after a sufficient period of immobilization will lead to improved insertion site mechanical and structural properties compared to immobilization alone. A period of immobilization will be necessary to 'protect' the insertion site and to allow for appropriate extracellular matrix (ECM) expression (e.g., type and III collagen, aggrecan, decorin and biglycan) such that the insertion site can re-form. Thus, the positive effect of remobilization requires a minimum period of immobilization. The specific aims are: 1) Following repair, immobilize shoulders either continuously, or with passive motion, and compare insertion site mechanics, collagen fiber orientation and ECM gene expression at 2, 6, 10, 14, 18 and 22 weeks post-repair, 2) Following repair, immobilize shoulders for 2, 6 and 10 weeks, then remobilize for 4, 8 and 12 weeks and compare insertion site mechanics, fiber orientation, and ECM gene expression to immobilization only at matching post-repair time points as well as over time, and, since immobilization has been shown to reduce range of motion, 3) Compare passive range of motion (ROM) prior to repair, immediately after immobilization has been discontinued, and after 3 days, 1, 2 and 6 weeks of remobilization. [unreadable] [unreadable]